1. Field of the Invention
The present invention relates generally to an anthracene derivative which can be used as a fluorescent material, light-emitting material, host material, carrier transport material or carrier injection material for holes or electrons used in an organic electroluminescent (hereinafter abbreviated as “organic EL”) device and display, and to an organic electroluminescent device using these materials.
2. Description of the Related Art
Organic electroluminescent devices are those which are provided with a solid or liquid light-emitting layer containing a fluorescent or phosphorescent organic luminescent material as one layer between electrodes facing each other and emit light by applying a voltage across the electrodes. Generally, a device containing a solid organic light-emitting layer is called an organic electroluminescent device and a device containing a liquid light-emitting layer is called an organic electroluminescent device or an organic electrochemical luminescent device. Broadly speaking, a device constituted of an organic light-emitting material that emits light by electric energy is an organic electroluminescent device, and such organic electroluminescent devices can include the device containing a liquid light-emitting layer.
Generally, an organic electroluminescent device having a solid light-emitting layer is provided with at least a light-emitting layer between a positive electrode and negative electrode which comprises a low-molecular-weight or polymer organic material that emits fluorescence or phosphorescence with high luminous efficiency.
The Light-emitting layer is generally sandwiched between a hole transport layer and an electron transport layer and further sandwiched between the positive electrode and negative electrode.
As the positive electrode, a transparent conductive film such as indium-tin complex oxide (hereinafter abbreviated as “ITO”) and indium-zinc complex oxide (hereinafter abbreviated as “IZO”) having a large ionization potential (hereinafter abbreviated as “Ip”) is used to reduce the energy barrier to inject holes into the organic light-emitting layer. There is the case where an oxide of tungsten, vanadium, molybdenum, ruthenium, titanium etc. are disposed on the surface of the positive electrode or blended in a material of the transparent conductive film to obtain a larger Ip.
As the negative electrode, a metal layer containing an alkali metal, alkali earth metal, or rare earth elements which have a low ionization potential and hence have a low electron injection barrier is used. However, they have the problem that it is necessary to sealed strictly because they are easily corroded by water.
General organic electroluminescent device has rectification characteristics and when a direct voltage is applied to this organic electroluminescent device, holes and electrons are transported from the positive electrode and negative electrode to the light-emitting layer through the hole transport layer and electron transfer layer, respectively, then, these holes and electrons are recombined with each other in the light-emitting layer to emit light. The organic electroluminescent device emits light when a forward bias is applied in the case of alternating voltage drive.
The luminous efficacy of an organic electroluminescent device is improved by keeping a carrier balance between holes and electrons to be injected. Further, it is effective to prevent carriers from being accumulated at the interface and inside of the organic layer for inhibition of deterioration and extension of life of the organic layer. In light of this, the film thickness and carrier transport ability of each layer and energy barrier of interlayer are regulated to manufacture organic electroluminescent device having a high efficiency and long-life.
As to the light-emitting mechanism of an organic electroluminescent device using a solution as the light-emitting layer, radical anions of a light-emitting material which are generated by the injection of electrons from the negative electrode and radical cations of a light-emitting material which are generated by the deprivation of electrons by the positive electrode were diffused in a solution and collided with each other, causing the light-emitting material to enter into an excited state to emit light. Although a luminance of hundreds to thousands of candelas (cd/m2) is obtained, the organic electroluminescent device has a problem concerning a too retarded response speed for use as a display, due to ionic diffusion-based conductance.
Hamada et al. dissolved about 0.5 wt % of a fluorescent compound having condensed aromatic rings such as rubrene in a mixed organic solvent of orthodichlorobenzene and toluene and added 1,2-diphenoxyethane as a positive ion conductive assist dopant to obtain a luminance of 100 cd/m2 at 80 V (Patent reference 1 and Non-patent reference 1).
Enomoto et al. dissolved rubrene and tetra-n-butylammoniumhexafluorophosphate as a supporting electrolyte in a mixed solvent of orthodichlorobenzene and acetonitrile (3:1) and applied alternating voltage drive under the condition of 30 Hz and ±1.9 V by using a comb-shaped electrode having a width of 8 μm and a gap of 4 μm, with the result that the positive electrode and the negative electrode are switched therebetween in the same electrode, so that both anionic and cationic radicals are generated around the electrode and recombined, which provides light emission with high efficiency and a luminance of 220 cd/m2 at a response speed of 3 ms (Non-patent reference 2).
Although organic electroluminescent devices of electrochemiluminescence type have an advantage that a stable metal can be used as the electrode material, they have the problem that they each have a shorter life than all-solid type organic electroluminescent devices due to the influence of oxygen, water and impurities in the solution and side reaction with the supporting electrolyte. However, these organic electroluminescent devices have been improved, for example, by using sparingly volatile ionic liquids as the mediums, by using a solid electrolyte to prevent deterioration, or by using a porous electrode to increase the surface area of the electrode, thereby developing electroluminescent devices having high luminance. As the light-emitting materials, phosphorescent materials besides low-molecular-weight fluorescent materials and polymer materials may be used and studies have been continued as to developments of the organic electroluminescent devices which have long life, high luminance and high efficiency. Recently, Add-vision Inc. have reported that an organic electroluminescent device which generate white color light of initial luminance of 100 cd/m2 and a long life exceeding 6000 hr (half-life) (Non-patent reference 3).
Hereinafter, a general organic electroluminescent device using a solid light-emitting layer will be explained in more detail.
Typically, an organic electroluminescent device has a structure in which layers such as a hole injection layer, an electron-block hole transport layer, a light-emitting layer, a hole-block electron transport layer, an electron transport layer, an electron injection layer, and a negative electrode are formed in this order on a transparent positive electrode formed on a transparent substrate such as glass and hermetically sealed.
Red, blue, or green light emission can be obtained easily by changing the molecular structure of the organic material contained in the light-emitting layer.
A method for obtaining white light emission includes, for example, a method in which a bluish green light-emitting material is doped with a yellowish orange light-emitting material to obtain a wide range spectrum, a method in which two light-emitting layers consisting of bluish green and yellow layers are stacked, a method in which structural units of red, green and blue light-emitting layers are stacked through a carrier-generating layer, and a method in which using an excimer light-emitting material, a bluish green monomer light emission is overlapped on a yellow to red color excimer light emission spectrum.
Examples of a system used to develop a color display of an organic electroluminescent device include a method in which red, blue, and green colors are separately applied to each pixel, a method in which a red, blue, green, or white color filter is superposed on a white light-emitting device to obtain spectral colors, and a method in which a film of, for example, a polymer electroluminescent material is formed on a blue or ultraviolet light-emitting device to obtain green or red color emission as a fluorescent conversion film (Non-patent reference 4).
As a method of forming the light-emitting layer, various methods have been developed. Though a film of a low-molecular-weight material is generally formed by vacuum deposition, the coating or printing method which is usually carried out for polymer materials may be applied to materials which are highly soluble and have a wet film-forming ability.
As a method of selective vapor deposition of pixels in different colors when using a low-molecular-weight material, a mask deposition method is generally used.
However, the mask deposition method has the problem concerning alignment accuracy between the mask and the substrate when larger size of substrate is used. The problem is due to the influences of difference in the thermal expansion between mask and the glass, application error of the mask to the frame, or deflection of the substrate and mask due to gravitation.
In light of this, laser transfer methods including a laser thermal transfer method and laser sublimation transfer method are attempted to improve alignment accuracy.
The laser thermal transfer method includes a LITI (Laser Induced Thermal Imaging) method (Non-patent reference 5) performed in 3M and Samsung. The laser sublimation transfer method includes a RIST (Radiation Induced Sublimation Transfer) method (Non-patent reference 6) method performed in Eastman Kodak and a LIPS (Laser Induced Pattern wise Sublimation) method (Non-patent reference 7) method performed in Sony.
In the LITI method, a donor film formed by depositing or applying a transfer material made of a low-molecular weight or polymer electroluminescent material is brought into close contact with the display substrate and irradiated with a laser according to a predetermined pixel in each color to perform thermal transfer. A 302 ppi highly precise display of 2.65 inches and a display of 17 inches have been produced experimentally. However, these displays pose the problem that a film having high film strength formed by a polymer electroluminescent material has deterioration in film cutting properties and generates burrs easily when peeling the donor film, and therefore, a low-molecular weight material which is soluble and has good wet film formation characteristics is also added.
In the RIST method, a film of a low-molecular-weight organic electroluminescent material is formed on a donor film by the vapor deposition method. After the formed film surface of the donor film and the display substrate are made to face each other at a small distance in vacuo, a laser is applied to a pixel having a target color from the backside of the donor film to deposit the pixel onto the display substrate by sublimation or vaporization.
In the LIPS method, a highly precise organic electroluminescent display of 27 inches is manufactured experimentally by using a transfer substrate made of glass having high registration characteristics in place of the donor film. However, it entails a high cost to form a film on a donor film or substrate by vapor deposition. A material which has a low molecular weight and can be formed by application is desired to reduce costs.
In the case of a low-molecular weight material or polymer material which is soluble in a solvent and can be formed in a wet system, highly precise selective vapor deposition of pixels in different colors can be accomplished by, for example, the inkjet method (Non-patent references 8 to 11), continuous nozzle printing method (Non-patent references 12 and 13), or relief printing method (Non-patent reference 14).
The inkjet method is a method enabling selective vapor deposition in an inexpensive apparatus even if the substrate is increased in size, and a polymer electroluminescent material is generally used in this method.
As a polymer blue light-emitting material, for example, copolymer consisted of fluorenone or phenoxazine and polyphenylene type copolymers, which each have an energy difference between ionization potential and electronic affinity (hereinafter abbreviated as Eg) larger than about 3 eV or more are used as the base. Green or red light-emitting polymer materials are synthesized by copolymerizing a blue light-emitting material as the base with a monomer which can emit green or red light and has a small Eg. A low-molecular weight type phosphorescent material having high luminous efficiency is added or introduced into the side chain of these light-emitting compounds.
Polymer materials tend to generate excimer emission light having a long wavelength by heating during the process of manufacturing a device or the operation of the device and it is particularly necessary to take care in molecular design to limit reduction in the color purity of EL light emission. Further, polymer materials cannot be refined by sublimation and are therefore highly purified with difficulty. Because the molecular weight is changed by the influence of water or impurities in the monomer, the polymer material has the problem concerning, for example, difficulty in the stability quality depending on the lot.
As a low-molecular weight blue light-emitting material, Ito (Toppan Printing Co., Ltd.) (Patent reference 2) and Jianmin Shi and Ching W. Tang etc., (Kodak) (Non-patent reference 15) developed unbipolar 9,10-di(2-naphthyl)-anthracene derivatives containing anthracene as a skeleton. These derivatives were used as blue light-emitting layer host materials for displays of, for example, MP3 players, personal multiplayers, and digital cameras.
9,10-di(2-naphthyl)-2-tertiarybutylanthracene (hereinafter abbreviated as “TBDNA”) which is a typical material of these derivatives has a glass transition temperature (hereinafter abbreviated as “Tg”) of about 128° C. when measured by differential scanning calorimeter (hereinafter abbreviated as DSC). Further, this material is crystallized at 222° C. or more and melts at 285° C. This leads to the case where the inside of the device is heated to Tg or more, causing mixing or crystallization of the organic layer, which develops short circuits when light with high luminance is emitted at a high current density. Therefore, a higher Tg and improvement in amorphous characteristics are desired.
Further, TBDNA can be formed in a wet system by the spin coating method using toluene. However, TBDNA has a molecular weight as low as 486 and therefore, the formed film is gradually sublimated even in vacuum drying at a temperature as relatively low as about 130° C., giving rise to a problem concerning insufficient heat resistance.
When the film is doped with 2,5,8,11-tetra-t-butylperylene for improving luminance, the EL light emission peak is shifted to the longer wavelength side, that is, from 460 nm to 465 nm, posing the problem in television use for which high color purity is required.
With regard to the drive system of an EL display, when a small and less precise display having a low display capacity is used, a passive matrix drive system is used as the driving system and the structure is changed to a bottom emission type in which light is extracted from the transparent substrate side to cope with this situation. However, a large current flows in the passive matrix drive system when each pixel dot emits light in the case of a display having a large display capacity such as a highly precise color television. For this, usual organic electroluminescent materials having Tg of about 100 to 150° C. are inferior in heat resistance, posing the problem that they fail to perform long time operation.
In light of this, the operation performed by an active matrix driving circuit using a thin film transistor driving circuit formed on a glass substrate or a CMOS driving circuit formed on a silicon wafer has come to be used to drive each pixel dot at as low a current density as possible.
Moreover, a top emission system is adopted in which an organic electroluminescent device is formed on an insulation film formed on a driving circuit and a light transmitting counter electrode is formed on the side opposite to the substrate to extract light, thereby enabling increase in ratio of aperture, and further enabling low-current density drive. Further, the top emission system enables strong extraction of light having a specific wavelength of an EL light emission spectrum by utilizing a micro-interference (microcavity) effect produced between the transparent positive electrode side reflection film and semitransparent negative electrode with an organic EL medium layer being sandwiched therebetween. Moreover, color filters may be laminated to thereby more improve color purity (Patent reference 3). However, the top emission system has a problem as to high manufacture costs.
The color purity can also be improved by developing a light-emitting host material and light-emitting dopant material which each emit light having a shorter wavelength through molecular design. Ito et al. synthesized, for example, 9,10-di(biphenyl-2-yl)-2-t-butylanthracene (hereinafter abbreviated as “TBBPA”) having an emission peak of about 420 nm and 450 nm (Patent reference 4). A CIE 1931 xy chromaticity coordinate of an electroluminescent device using TBDNA as a light-emitting layer was (0.172, 0.183). A device using TBBPA was so improved that the xy chromaticity becomes (0.157, 0.128). However, the glass transition temperature of TBBPA is as low as about 74° C., posing the problem concerning the necessity of improvement in the heat resistance of TBBPA.
Further, in the molecular design of a diamine type low-molecular weight hole transport material, a material having high amorphous characteristics and high solubility in an organic solvent can be obtained by introducing non-symmetry into a molecule (Non-patent reference 16). Similarly, a non-symmetrical structure is introduced into an anthracene type blue light-emitting material and there is also an attempt to develop a coating type electroluminescent device using a low-molecular weight material. However, many materials have a solubility of 2 wt % or less in toluene and the solubility of a material is in the order of about 5 wt % even if the material has higher solubility in toluene (Patent references 5 to 7). Further, the coating type electroluminescent device has a shorter life than the vapor deposition type.
Because of this, it is desired to develop a new lower-molecular weight light-emitting material having high color purity, high heat resistance, high solubility, high wet film forming characteristics, and high durability.
PRIOR ART REFERENCE
Patent Reference
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